Zoom lens system

ABSTRACT

The invention relates to a positive lens group preceding type of compact zoom lens system for use on still cameras, in which the focal distance at the wide-angle end is shorter than the diagonal length of an image pickup surface, the angle of field is at least 64° at the wide-angle end and the zoom ratio is at least 3, and which comprises a first positive lens group G1, a second negative lens group G2, a third positive lens group G3 and a fourth positive lens group G4 so that the first, third and fourth lens groups G1, G3 and G4 are moved toward the object side for zooming from the wide-angle end to the telephoto end. The second lens group G2 comprises three lenses, the fourth lens group G4 comprises a positive lens component and a negative lens component, and the third and fourth lens groups G3 and G4 have at least two aspheric surfaces, with conditional inequality 0.1&lt;d 4  /f W  &lt;0.5 being satisfied, where d 4  is the air separation between the positive and negative lens components of said fourth lens group and f W  is the composite focal distance of the overall system at the wide-angle end.

BACKGROUND OF THE INVENTION

The present invention relates generally to a zoom lens, and moreparticularly to a zoom lens system suitable for use on still cameras,which has a zoom ratio of at least 3 and in which the focal length atthe wide-angle end is shorter than the diagonal length of an imagepickup surface.

Zoom lenses for use on recently developed function-intensive still andother cameras are increasingly required to be compact and have a zoomratio high enough to enable a single lens to cover an area from thewide-angle end, having a wider angle of field, even to the telephotoend, having a long focal distance.

In general, zoom lenses are broken down into two types, one called apositive lens group preceding type (meaning that the foremost lens grouphas a positive refracting power) and the other a negative lens grouppreceding type (meaning that the foremost lens group has a negativerefracting power). Of these two types, the negative lens group precedingtype is beneficial for enlarging the angle of field at the wide-angleend. At a zoom ratio of 2 or more, however, there is an increase in theoverall length of the lens system. Moreover, since light flux isdiverged by the negative lens group, there is an increase in thediameter of the lens group(s) that follow. Thus, it is difficult to makea reasonable compromise between obtaining a high zoom ratio and makingthe lens diameters small.

The positive lens group preceding type, on the other hand, is inferiorto the negative lens group preceding type in terms of enlarging theangle of field at the wide-angle end. Since light flux is converged bythe positive lens group, however, this lens type enables the effectivediameter of the lens group(s) that follow it to be decreased and a highzoom ratio can obtained. In this respect, it is superior to the formertype.

In a positive lens group preceding type of zoom lens system containing awide-angle area where the focal distance at the wide-angle end isshorter than the diagonal length of an image pickup surface, and has azoom ratio of at least about 3, a strong negative refracting power isimparted to the second lens group thereof and a lens group or groupshaving a positive refracting power are located off the second lens grouptoward the image side. For zooming, it is general that separationsbetween the first lens group having a positive refracting power and thesecond lens group having a negative refracting power and optionally alens group or groups located off the second lens group toward the imageside, are varied.

In order for the lens system of such a design to be used to obtain ahigh zoom ratio and reduce the total length and amount of movement ofthe lens groups during zooming, thereby achieving compactness, therefracting powers of the lens groups (especially that of the second lensgroup having a negative refracting power) must be increased.

In the zoom lens system containing a wide-angle area where the focaldistance at the wide-angle end is shorter than the diagonal length of animage pickup surface and having a zoom ratio of at least about 3, theangle of field varies greatly between the wide-angle end and thetelephoto end. Consequently, the second lens group is generally made upof three negative lenses and one positive lens to accommodate itself toa change of the angle of incidence of off-axis rays incident thereon, asdisclosed in JP-A 63-66522, JP-A 63-66523, JP-A 63-294506, and U.S. Pat.No. 4,871,239.

JP-A 54-30855, JP-A 55-156912, JP-A 1-178912 and JP-A 6-34886 discloseexamples of the positive lens group preceding type wherein the secondlens groups having a negative refracting power are each made up of threelenses, negative, negative and positive in that order from the objectside.

Furthermore, JP-A 4-317019 discloses an example of the positive lensgroup preceding type wherein at least three aspheric surfaces are usedthroughout the overall system to achieve reductions in the number oflenses, cost reductions and compactness while high optical propertiesare maintained.

In the zoom lens systems disclosed in JP-A 63-66522, JP-A 63-66523, JP-A63-294506 and U.S. Pat. No. 4,871,239, each of the second lens groupshaving a negative refracting power is made up of a negative lens, anegative lens, a positive lens and a negative lens in that order fromthe object side, thereby accommodating itself to the strong negativerefracting power and a change of the angle of incidence of off-axisrays. However, these lens designs are still less than satisfactory interms of cost and size because as many as four lenses are used for thesecond lens groups.

Some of the above-mentioned publications disclose examples wherein anaspheric surface is introduced in the second lens groups having anegative refracting power for the purpose of correcting for distortionand the second lens groups are all made up of four lenses. However,these references teach how to reduce the number of lenses to three orless.

Moreover, JP-A 54-30855, JP-A 55-156912 and JP-A 1-178912 refer toexamples of the positive lens group preceding type wherein the secondlens groups having a negative refracting power are each made up of threelenses, negative, negative and positive in that order from the objectside. However, they have the problems that high zoom ratios are notachievable and the total lengths of the second lens groups consisting ofthree lenses are too long, and so are less than satisfactory regarding ahigh zoom ratio and compactness.

JP-A 6-34886 further discloses an example of the positive lens grouppreceding type with a zoom ratio of about 3.7, wherein the second lensgroup has a negative refracting power and is made up of a negative lens,a negative lens and a positive lens in that order from the object side.However, a wide enough angle of field is not achievable at thewide-angle end, as can be understood from the exemplified angle of fieldof as small as about 64° at the wide-angle end.

JP-A 4-317019 further discloses that at least three aspheric surfacesare used throughout the overall system to reduce the number of lensesconstituting each lens group to three or less. However, the zoom ratioexemplified there is 3 at most and so is still unsatisfactory.

SUMMARY OF THE INVENTION

In view of the above problems with the prior art devices, it istherefore an object of the present invention to provide a positive lensgroup preceding type of zoom lens system including a second lens grouphaving a negative refracting power and consisting of three lenses,wherein an aspheric surface or surfaces are effectively used, so thatwhile high optical properties are maintained, the number of lensesinvolved can be reduced to achieve cost reductions and compactness. Inthis zoom lens system, the focal distance at the wide-angle end isshorter than the diagonal length of an image pickup surface and, inparticular, the angle of field at the wide-angle end is at least 64°while the zoom ratio is at least 3, with well-corrected aberrations.

According to a first aspect of the present invention, theabove-mentioned object is achieved by providing a zoom lens system witha focal distance at the wide-angle end which is shorter than thediagonal length of an image pickup surface. The zoom lens systemaccording to this aspect of the invention comprises, in order from theobject side, a first lens group having a positive refracting power, asecond lens group having a negative refracting power, a third lens grouphaving a positive refracting power and a fourth lens group having apositive refracting power wherein the first, third and fourth lensgroups are moved toward the object side for zooming from the wide-angleend to the telephoto end. The zoom lens system is characterized in thatsaid second lens group comprises three lenses, said fourth lens groupcomprises, in order from the object side, a positive lens component anda negative lens component, and said third and fourth lens groups have atleast two aspheric surfaces. It is further characterized by conformingto the following conditional inequality:

    0.1<d.sub.4 /f.sub.W <0.5                                  (1)

where d₄ is the air separation between the positive and negative lenscomponents of said fourth lens group and f_(W) is the composite focaldistance of the overall system at the wide-angle end.

According to a second aspect of the present invention, a zoom lenssystem with the focal distance at the wide-angle end being shorter thanthe diagonal length of an image pickup surface is provided. The zoomlens system according to this aspect of the invention comprises, inorder from the object side, a first lens group having a positiverefracting power, a second lens group having a negative refractingpower, a third lens group having a positive refracting power and afourth lens group having a positive refracting power wherein the first,third and fourth lens groups are moved toward the object side forzooming from the wide-angle end to the telephoto end. The zoom lenssystem is characterized in that each of said lens groups comprises up tothree lenses, said fourth lens group comprises, in order from the objectside, a positive lens component and a negative lens component, and saidthird and fourth lens groups have at least two aspheric surfaces. It isfurther characterized by conforming to the following conditionalinequality:

    0.1<d.sub.4 /f.sub.W <0.5                                  (1)

where d₄ is the air separation between the positive and negative lenscomponents of said fourth lens group and f_(W) is the composite focaldistance of the overall system at the wide-angle end.

According to a third aspect of the present invention, a zoom lens systemwith the focal distance at the wide-angle end being shorter than thediagonal length of an image pickup surface is provided. The zoom lenssystem according to this aspect of the invention comprises, in orderfrom the object side, a first lens group having a positive refractingpower, a second lens group having a negative refracting power, a thirdlens group having a positive refracting power and a fourth lens grouphaving a positive refracting power wherein the first, third and fourthlens groups are moved toward the object side for zooming from thewide-angle end to the telephoto end. The zoom lens system ischaracterized in that said second lens group comprises three lenses, andsaid third and fourth lens groups have at least two aspheric surfaces.It is further characterized in that said first and second lens groupsare moved together for focusing.

According to a fourth aspect of the present invention, a zoom lenssystem with the focal distance at the wide-angle end being shorter thanthe diagonal length of an image pickup surface is provided. The zoomlens system according to this aspect of the invention comprises, inorder from he object side, a first lens group having a positiverefracting power, a second lens group having a negative refractingpower, a third lens group having a positive refracting power and afourth lens group having a positive refracting power wherein the first,third and fourth lens groups are moved toward the object side forzooming from the wide-angle end to the telephoto end. The zoom lenssystem is characterized in that said second lens group comprises, inorder from the object side, a negative meniscus lens convex on theobject side, a double-concave lens with the object-side surface having astronger curvature, and a double-convex lens with the object-sidesurface having a stronger curvature. The third lens group consistsessentially of, in order from the object side, a double-convex lens withthe object-side surface having a stronger curvature, a positive lenswith the object-side surface having a stronger curvature and a negativemeniscus lens convex on the image side. The fourth lens group comprisesa positive lens component and a negative lens component in order fromthe object side, said negative lens component comprises a negative lenswith the image-side surface concave on the image side, and said thirdand fourth lens groups have at least two aspheric surfaces.

A detailed account will now be given of why the above-mentionedarrangements are used and how they act.

Generally, to the positive lens group preceding type of zoom lens systemsuch as one according to the present invention which covers from awide-angle area having a wider angle of field to a telephoto area havinga longer focal distance (i.e.,); contains a wide-angle end area wherethe focal distance is shorter than the diagonal length of an imagepickup surface, and has a zoom ratio of at least about 3, overallconstitutes a retrofocus type at the wide-angle end where the secondlens group is closest to the first lens group and a telephoto type atthe telephoto end where the second lens group is farthest from the firstlens group. Here the problem that must be solved to make the angle offield wider at the wide-angle end and achieve a higher zoom ratio aswell as to make the overall length of the lens system from thewide-angle end shorter to the telephoto end is how efficiently therefracting power of the second negative lens group is increased.However, if the refracting power of the second negative lens group wereto be increased to make the angle of field at the wide-angle end wideand make the zoom ratio of the lens system high simultaneously withachieving compactness, various aberrations produced by the second lensgroup would become too large. In addition, the negative Petzval sum ofthe lens system would increase overall, resulting in the tilting offield toward the image side. Cost reductions may be achieved by reducingthe number of lenses forming the second lens group, but this becomes abarrier to correcting for various aberrations.

The above-mentioned object is achieved by the first embodiment of thepresent invention, according to which a zoom lens system with the focaldistance at the wide-angle end being shorter than the diagonal length ofan image pickup surface is provided. The zoom lens system according tothis aspect of the invention comprises, in order from the object side, afirst lens group having a positive refracting power, a second lens grouphaving a negative refracting power, a third lens group having a positiverefracting power and a fourth lens group having a positive refractingpower and in which the first, third and fourth lens groups are movedtoward the object side for zooming from the wide-angle end to thetelephoto end. The zoom lens system is characterized in that said secondlens group comprises three lenses, said fourth lens group comprises, inorder from the object side, a positive lens component and a negativelens component. The third and fourth lens groups have at least twoaspheric surfaces, and are further characterized by conforming to thefollowing conditional inequality:

    0.1<d.sub.4 /f.sub.W <0.5                                  (1)

where d₄ is the air separation between the positive and negative lenscomponents of said fourth lens group and f_(W) is the composite focaldistance of the overall system at the wide-angle end. The refractingpower of the second negative lens group is increased, especially to makethe zoom ratio of the positive lens group preceding type of zoom lenssystem high and to make the angle of field thereof at the wide-angle endwide. However, if the number of lenses forming the second lens groupwere to be reduced from four (as conventional) to three, it would bedifficult to make satisfactory correction for various aberrations overthe total focal distance area. It is therefore desired to makesatisfactory correction for spherical aberration over the total focaldistance area by use of at least one aspheric surface for the surface ofthe third and fourth lens group that is relatively close to a stop andto make satisfactory correction for curvature of field and coma by useof at least one aspheric surface for the lens group that is relativelyclose to the image surface.

Conditional inequality (1) is given to limit a lowering of theaberration performance of the fourth lens group with respect todecentering. If the upper limit of 0.5 is exceeded, the lens diameter ofthe fourth lens group increases, thus making it difficult to make theoverall lens system compact, although the lowering of the aberrationperformance of the fourth lens group with respect to decentering may belimited.

If the lower limit of 0.1 is not reached, it is difficult for theoverall system to maintain satisfactory aberration performance while thelowering of the aberration performance with respect to decentering islimited.

The second embodiment of the zoom lens system according to the presentinvention comprises, in order from the object side, a first lens grouphaving a positive refracting power, a second lens group having anegative refracting power, a third lens group having a positiverefracting power and a fourth lens group having a positive refractingpower wherein the first, third and fourth lens groups are moved towardthe object side for zooming from the wide-angle end to the telephotoend. The zoom lens system is characterized in that each of said lensgroups comprises up to three lenses. The fourth lens group comprises apositive lens component, and a negative lens component in that orderfrom the objet side, and said second, third and fourth lens groups haveat least two aspheric surfaces, and are further characterized byconforming to the above-mentioned conditional inequality (1).

The refracting powers of the following lens groups would increase,especially if the refracting power of the second lens group were to beincreased so as to make the zoom ratio of the lens system highsimultaneously with making the angle of field thereof at the wide-angleend wide. If the number of lenses forming each lens group were to be notmore than three, on the other hand, it would be difficult to makesatisfactory correction for various aberrations over the total focaldistance area. To achieve cost reductions and compactness while thenumber of lenses forming each lens groups is reduced to three or less,it is therefore desired to make satisfactory correction for sphericalaberration over the total focal distance area by use of at least oneaspheric surface for the surface of the third and fourth lens groupsthat is relatively close to the stop and make satisfactory correctionfor curvature of field and coma over the total focal distance area,while higher-order aberrations produced by the second lens group andthose produced by the following lens groups are in a well-balancedstate, by use of at least one aspheric surface for the surface of thethird and fourth lens groups that is relatively close to the image side.By conforming to conditional inequality (1) it is possible to limit thelowering of the aberration performance of the fourth lens group withrespect to decentering.

The third embodiment of the zoom lens system according to the presentinvention comprises, in order from the object side, a first lens grouphaving a positive refracting power, a second lens group having anegative refracting power, a third lens group having a positiverefracting power and a fourth lens group having a positive refractingpower wherein the first, third and fourth lens groups are moved towardthe object side for zooming from the wide-angle end to the telephotoend. The zoom lens system is characterized in that said second lensgroup comprises three lenses, and said third and fourth lens groups haveat least two aspheric surfaces. Furthermore, the zoom lens system ischaracterized in that said first and second lens groups are movedtogether for focusing.

According to the third embodiment of the present invention, compactnessis achieved by reducing the number of lenses involved while higher zoomratios and wider angles of field are achieved, as already mentioned.Moreover, the above-mentioned focusing mechanism is suitable forreducing the shortest object distance during focusing while achievingcompactness.

In a prior art positive lens group preceding type of zoom lens system inparticular, focusing is done by drawing out the front lens. Thisfocusing mechanism is advantageous in that the amount of the front lensto be drawn out is kept constant irrespective of the zooming positionbut, with this, it is impossible to reduce a near-by object distancebecause, upon the front lens being drawn out, an off-axis bundle isshaded by the amount that the front lens is drawn out, resulting in adrop of off-axis field illuminance. If the diameter of the front lenswere to be increased so as to prevent the shading of the off-axisbundle, the lens system would increase in size.

In the present invention, the first and second lens groups are movedtogether toward the object side for focusing from an infinite objectdistance to a near-by object distance. With this focusing mechanism itis possible to reduce the amount of focusing movement, because thecomposite refracting power of the first and second lens groups is largerthan that of the front lens (the first lens group). This in turn enablesthe near-by object distance to be reduced and in particular, makes itpossible to reduce the amount of focusing movement at the wide-angleend; so the front lens can be much more reduced in diameter.

This focusing mechanism is also beneficial to correcting for aberrationsduring focusing from the point at infinity to a near-by object distance,because variations of spherical aberration and curvature of field arelimited. In other words, when focusing is done by drawing out the frontlens from a focusing state, where spherical aberration and curvature offield are corrected for the object point at infinity, with respect to anear-by object point, the spherical aberration and the curvature offield vary in opposite directions. According to the above-mentionedfocusing mechanism involving the integral movement of the first andsecond lens groups, both the spherical aberration and the curvature offield vary in the same direction even when focusing is done from thefocusing state, where such aberrations are corrected for the objectpoint at infinity, with respect to a near-by object point. This isbeneficial to correction of aberrations, and is advantageous forreducing the near-by object distance as well.

According to the fourth embodiment of the present invention, a zoom lenssystem with the focal distance at the wide-angle end being shorter thanthe diagonal length of an image pickup surface is provided. The zoomlens system according to this aspect of the invention comprises, inorder from the object side, a first lens group having a positiverefracting power, a second lens group having a negative refractingpower, a third lens group having a positive refracting power and afourth lens group having a positive refracting power wherein the first,third and fourth lens groups are moved toward the object side forzooming from the wide-angle end to the telephoto end. The zoom lenssystem is characterized in that said second lens group comprises, inorder from the object side, a negative meniscus lens convex on theobject side, a double-concave lens with the object-side surface having astronger curvature, and a double-convex lens with the object-sidesurface having a stronger curvature. The third lens group comprises, inorder from the object side, a double-convex lens with the object-sidesurface having a stronger curvature, a positive lens with theobject-side surface having a stronger curvature and a negative meniscuslens convex on the image side. The fourth lens group comprises apositive lens component and a negative lens component in order from theobject side, said negative lens component comprises a negative lens withthe image-side surface concave on the image side, and said third andfourth lens groups have at least two aspheric surfaces.

According to this embodiment of the present invention, compactness isachieved while higher zoom ratios and wider angles of field areaccomplished, and satisfactory aberration performance is obtained forthe overall lens system.

Furthermore, the zoom lens system according to the present invention ischaracterized by conforming to the following inequalities:

    -0.33<f.sub.12W /f.sub.T <-0.1                             (2)

    0.01<Δd.sub.3 /f.sub.W <0.35                         (3)

    -0.33<f.sub.12W /f.sub.T <-0.2                             (2')

    0.01<Δd.sub.3 /f.sub.W <0.25                         (3')

Here f_(12W) is the composite focal distance of the first and secondlens groups at the wide-angle end, f_(T) is the composite focal distanceof the overall lens system at the telephoto end, Δd₃ is the value foundby subtracting the air separation between the third and fourth lensgroups at the telephoto end from the air separation between the thirdand fourth lens groups at the wide-angle end, and f_(W) is the compositefocal distance of the overall lens system at the wide-angle end.

Conditional inequality (2) is given to limit the total length of thelens system at the wide-angle end. At the wide-angle end where the firstand second lens groups are closest to each other, they constitute,overall, a retrofocus type of front lens group having a negativerefracting power. When the lower limit of inequality (2) does not reach-0.33, the composite refracting power of the first and second lensgroups which overall yield a negative refracting power becomes too smallto make the lens system compact. When the upper limit of -0.1 isexceeded, the composite refracting power of the first and second lensgroups becomes too large to correct for aberrations produced there, inparticular, negative distortion at the wide-angle end.

To reduce the total length of the lens system at the wide-angle end, itis further advantageous to satisfy conditional inequality (2').

Conditional inequality (3) is set down to make satisfactory correctionfor curvature of field over the total zooming area. Falling below thelower limit of 0.01 is not preferable because the image surface tiltslargely in the positive direction during zooming from the wide-angle endto the telephoto end. Exceeding the upper limit of 0.35 is again notpreferable because the image surface tilts largely in the negativedirection. There is also an increase in the separation between the thirdand fourth lens groups, which is in turn disadvantageous for making thetotal length of the lens system compact.

To make more satisfactory correction for curvature of field over thetotal zooming area, it is further advantageous to satisfy inequality(3').

Still other objects and advantages of the present invention will in partbe obvious and will in part be apparent from the specification.

The present invention accordingly comprises the features ofconstruction, combinations of elements, and arrangement of parts whichwill be exemplified in the construction hereinafter set forth, and thescope of the present invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C are sectional views of one lens group arrangement of thezoom lens system of Example 1 according to the present invention at thewide-angle end, an intermediate focal distance, and the telephoto end,respectively

FIGS. 2A-2C are sectional views, similar to FIGS. 1A-1C, of the zoomlens system of Example 2 according to the present invention,

FIGS. 3A-3C are sectional views, similar to FIG. 1, of the zoom lenssystem of Example 3 according to the present invention, and

FIGS. 4A-4C are sectional views, similar to FIGS. 1A-1C, of the zoomlens system of Example 4 according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Examples 1-4 of the zoom lens system according to the present inventionwill now be explained with reference to the drawings.

FIGS. 1A to 4C illustrate, in section, four arrangements of the lensgroups of Examples 1 to 4 at the wide-angle end, an intermediate focaldistance, and the telephoto end.

In regard to Example 1 as shown in FIGS. 1A-1C, the zoom lens system ismade up of, in order from the object side, a first lens group G1 that isa doublet consisting of a negative meniscus lens concave on the imageside and a positive meniscus lens convex on the object side, a secondlens group G2 consisting of a negative meniscus lens concave on theimage side, a double-concave negative lens and a double-convex positivelens, a stop S, a third lens group G3 consisting of a double-convexpositive lens, a positive meniscus lens convex on the object side and anegative meniscus lens convex on the image side, and a fourth lens groupG4 consisting of a double-convex positive lens and a negative meniscuslens concave on the image side. It is here to be noted that theobject-side surface of the double-convex lens in the third lens group G3and the object-side surface of the negative meniscus lens concave on theimage side, incorporated in the fourth lens group G4, are aspheric.

In regard to Example 2 as shown in FIGS. 2A-2C, the zoom lens system ismade up of, in order from the object side, a first lens group G1consisting of a doublet of a plano-concave lens concave on the imageside and a double-convex positive lens, and a positive meniscus lensconvex on the object side, a second lens group G2 consisting of anegative meniscus lens concave on the image side, a double-concavenegative lens and a double-convex positive lens, a stop S, a third lensgroup G3 consisting of a double-convex positive lens, a positivemeniscus lens convex on the object side and a negative meniscus lensconvex on the image side, and a fourth lens group G4 consisting of adouble-convex positive lens, a negative meniscus lens concave on theimage side and a positive meniscus lens convex on the object side. It ishere to be noted that the object-side surface of the double-convex lensin the third lens group G3 and the object-side surface of the negativemeniscus lens concave on the image side, incorporated in the fourth lensgroup G4, are aspheric.

In regard to Example 3 as shown in FIGS. 3A-3C, the zoom lens system ismade up of, in order from the object side, a first lens group G1consisting of a doublet of a negative meniscus lens concave on the imageside and a positive meniscus lens convex on the object side, a secondlens group G2 consisting of a negative meniscus lens concave on theimage side, a double-concave negative lens and a double-convex positivelens, a stop S, a third lens group G3 consisting of a double-convexpositive lens, a positive meniscus lens convex on the object side and anegative meniscus lens convex on the image side, a fourth lens group G4consisting of a double-convex positive lens and a double-concavenegative lens, and a fifth lens group G5 consisting of a negativemeniscus lens concave on the image side. It is here to be noted that theobject-side surface of the double-convex lens in the third lens group G3and the object-side surface of the double-concave negative lens in thefourth lens group G4 are aspheric.

Finally, in regard to Example 4 as shown in FIGS. 4A-4C, the zoom lenssystem is made up of, in order from the object side, a first lens groupG1 consisting of a doublet of a negative meniscus lens concave on theimage side and a double-convex positive lens, and a positive meniscuslens convex on the object side, a second lens group G2 consisting of anegative meniscus lens concave on the image side, a double-concavenegative lens and a double-convex positive lens, a stop S, a third lensgroup G3 consisting of a double-convex positive lens, a positivemeniscus lens convex on the object side and a negative meniscus lensconvex on the image side, and a fourth lens group G4 consisting of adouble-convex positive lens and a plano-concave negative lens. It ishere to be noted that the object-side surface of the double-convexpositive lens in the third lens group G3 and the object-side surface ofthe negative lens in the fourth lens group G4 are aspheric.

In each example, focusing is done by drawing out the first and secondlens groups G1 and G2 toward the object side as an integral piece.

Set out below are numerical data on each example. Symbols usedhereinafter defined but not hereinbefore mean:

F_(NO) F-number,

f_(B) Back focus

2ω Angle of field

r₁, r₂, Radius of curvature of each lens surface

d₁, d₂, Separation between respective lens surfaces

n_(d1), n_(d2), d-line index of refraction of each lens,

ν_(d1), ν_(d2), Abbe's number of each lens

Now let x and y denote the direction of propagation of light on theoptical axis and the direction perpendicular to the optical axis,respectively. Then, aspheric configuration is given by:

    x=(y.sup.2 /r)/ 1+{1-(y/r).sup.2 }.sup.1/2 !+A.sub.4 y.sup.4 +A.sub.6 y.sup.6 +A.sub.8 y.sup.8 +A.sub.10 y.sup.10 +A.sub.12 y.sup.12

where r is the paraxial radius of curvature, and A₄, A₆, A₈, A₁₀ and A₁₂are the 4th, 6th, 8th, 10th and 12th aspherical coefficients. In eachexample, the diagonal length of the image pickup surface is 34.6 mm.

    __________________________________________________________________________    Example 1                                                                                f = 25.1 ˜ 50.0 ˜ 99.9                                            F.sub.NO = 4.52 ˜ 5.10 ˜ 5.60                                     f.sub.B = R ˜ 31.983 ˜ 47.513 ˜ 64.991                      2 ω= 71.96° ˜ 37.04° ˜                        18.92°                                                      r.sub.1 =                                                                        61.4439   d.sub.1 =                                                                        2.000                                                                              n.sub.d1 =                                                                       1.84666                                                                            υ.sub.d1 =                                                                23.78                                        r.sub.2 =                                                                        39.2527   d.sub.2 =                                                                        5.924                                                                              n.sub.d2 =                                                                       1.72916                                                                            υ.sub.d2 =                                                                54.68                                        r.sub.3 =                                                                        2220.0625 d.sub.3 =                                                                        (Variable)                                                    r.sub.4 =                                                                        2485.2879 d.sub.4 =                                                                        1.200                                                                              n.sub.d3 =                                                                       1.77250                                                                            υ.sub.d3 =                                                                49.60                                        r.sub.5 =                                                                        18.2763   d.sub.5 =                                                                        6.573                                                         r.sub.6 =                                                                        -32.8877  d.sub.6 =                                                                        1.200                                                                              n.sub.d4 =                                                                       1.77250                                                                            υ.sub.d4 =                                                                49.60                                        r.sub.7 =                                                                        88.2173   d.sub.7 =                                                                        0.200                                                         r.sub.8 =                                                                        45.9271   d.sub.8 =                                                                        2.903                                                                              n.sub.d5 =                                                                       1.84666                                                                            υ.sub.d5 =                                                                23.78                                        r.sub.9 =                                                                        -93.1287  d.sub.9 =                                                                        (Variable)                                                    r.sub.10 =                                                                       (Stop)    d.sub.10 =                                                                       1.000                                                         r.sub.11 =                                                                       28.9392(Aspheric)                                                                       d.sub.11 =                                                                       3.075                                                                              n.sub.d6 =                                                                       1.51633                                                                            υ.sub.d6 =                                                                64.15                                        r.sub.12 =                                                                       -102.6988 d.sub.12 =                                                                       0.200                                                         r.sub.13 =                                                                       20.1059   d.sub.13 =                                                                       2.892                                                                              n.sub.d7 =                                                                       1.48749                                                                            υ.sub.d7 =                                                                70.21                                        r.sub.14 =                                                                       95.0748   d.sub.14 =                                                                       2.884                                                         r.sub.15 =                                                                       -22.5250  d.sub.15 =                                                                       1.200                                                                              n.sub.d8 =                                                                       1.84666                                                                            υ.sub.d8 =                                                                23.78                                        r.sub.16 =                                                                       -84.5711  d.sub.16 =                                                                       (Variable)                                                    r.sub.17 =                                                                       32.9624   d.sub.17 =                                                                       3.288                                                                              n.sub.d9 =                                                                       1.65844                                                                            υ.sub..sub.d9 =                                                           50.86                                        r.sub.18 =                                                                       -47.3011  d.sub.18 =                                                                       5.535                                                         r.sub.19 =                                                                       158.6571(Aspheric)                                                                      d.sub.19 =                                                                       1.200                                                                              n.sub.d10 =                                                                      1.77250                                                                            υ.sub..sub.d10 =                                                          49.60                                        r.sub.20 =                                                                       29.7141                                                                    __________________________________________________________________________    Zooming Spaces                                                                f      25.1         50.0                                                                              99.9                                                  d.sub.3                                                                              1.0709       11.5645                                                                           26.3321                                               d.sub.9                                                                              26.6226      10.9719                                                                           1.4517                                                d.sub.16                                                                             5.4703       2.2374                                                                            0.9673                                                __________________________________________________________________________    Aspherical Coefficients                                                                     11th surface                                                                  A.sub.4 = 0.17242 × 10.sup.-4                                           A.sub.6 = 0.40330 × 10.sup.-7                                           A.sub.8 = 0.37834 × 10.sup.-9                                           A.sub.10 = 0.16198 × 10.sup.-13                                         A.sub.12 = 0                                                                  19th surface                                                                  A.sub.4 = 0.60612 × 10.sup.-4                                           A.sub.6 = 0.22036 × 10.sup.-6                                           A.sub.8 = 0.11638 × 10.sup.-8                                           A.sub.10 = 0.95002 × 10.sup.-11                                         A.sub.12 = 0                                                                  (1) d.sub.4 /f.sub.w = 0.22                                                   (2) f.sub.12W /f.sub.T = 0.31                                                 (3) AΔd.sub.3 /f.sub.w = 0.18                             __________________________________________________________________________    Example 2                                                                                f = 25.3 ˜ 50.0 ˜ 99.8                                            F.sub.NO = 4.66 ˜ 5.20 ˜ 5.74                                     f.sub.B = 31.988 ˜ 46.041 ˜ 60.285                                2 ω= 71.58° ˜ 36.96° = 18.94°                1                                                                  r.sub.1 =                                                                        ∞   d.sub.1 =                                                                        2.400                                                                              n.sub.d1 =                                                                       1.84666                                                                            υ.sub.d1 =                                                                23.78                                        r.sub.2 =                                                                        115.3975  d.sub.2 =                                                                        4.856                                                                              n.sub.d2 =                                                                       1.69680                                                                            υ.sub.d2 =                                                                55.53                                        r.sub.3 =                                                                        -172.0825 d.sub.3 =                                                                        0.200                                                         r.sub.4 =                                                                        49.7631   d.sub.4 =                                                                        3.810                                                                              n.sub.d3 =                                                                       1.69680                                                                            υ.sub.d3 =                                                                55.53                                        r.sub.5 =                                                                        111.1101  d.sub.5 =                                                                        (Variable)                                                    r.sub.6 =                                                                        78.9502   d.sub.6 =                                                                        1.200                                                                              n.sub.d4 =                                                                       1.77250                                                                            υ.sub.d4 =                                                                49.60                                        r.sub.7 =                                                                        16.1844   d.sub.7 =                                                                        6.586                                                         r.sub.8 =                                                                        -22.8810  d.sub.8 =                                                                        1.200                                                                              n.sub.d5 =                                                                       1.75250                                                                            υ.sub.d5 =                                                                49.60                                        r.sub.9 =                                                                        51.3956   d.sub.9 =                                                                        0.200                                                         r.sub.10 =                                                                       39.9198   d.sub.10 =                                                                       2.484                                                                              n.sub.d6 =                                                                       1.84666                                                                            υ.sub.d6 =                                                                23.78                                        r.sub.11 =                                                                       -66.3029  d.sub.11 =                                                                       (Variable)                                                    r.sub.12 =                                                                       ∞ (Stop)                                                                          d.sub.12 =                                                                       1.000                                                         r.sub.13 =                                                                       23.2661 (Aspheric)                                                                      d.sub.13 =                                                                       2.775                                                                              n.sub.d7 =                                                                       1.60311                                                                            υ.sub.d7 =                                                                60.70                                        r.sub.14 =                                                                       -240.7362 d.sub.14 =                                                                       0.200                                                         r.sub.15 =                                                                       21.8362   d.sub.15 =                                                                       2.674                                                                              n.sub.d8 =                                                                       1.49700                                                                            υ.sub.d8 =                                                                81.61                                        r.sub.16 =                                                                       868.0450  d.sub.16 =                                                                       2.241                                                         r.sub.17 =                                                                       -20.5162  d.sub.17 =                                                                       1.200                                                                              n.sub.d9 =                                                                       1.80518                                                                            υ.sub.d9 =                                                                25.43                                        r.sub.18 =                                                                       -127.4413 d.sub.18 =                                                                       (Variable)                                                    r.sub.19 =                                                                       45.4069   d.sub.19 =                                                                       3.074                                                                              n.sub.d10 =                                                                      1.67003                                                                            υ.sub.d10 =                                                               47.25                                        r.sub.20 =                                                                       -28.7830  d.sub.20 =                                                                       3.494                                                         r.sub.21 =                                                                       110.1787 (Aspheric)                                                                     d.sub.21 =                                                                       1.200                                                                              n.sub.d11 =                                                                      1.78590                                                                            υ.sub.d11 =                                                               44.19                                        r.sub.22 =                                                                       20.1863   d.sub.22 =                                                                       0.282                                                         r.sub.23 =                                                                       16.0292   d.sub.23 =                                                                       1.992                                                                              n.sub.d12 =                                                                      1.48749                                                                            υ.sub.d12 =                                                               70.21                                        r.sub.24 =                                                                       22.0705                                                                    __________________________________________________________________________    Zooming Spaces                                                                f      25.3         50.0                                                                              99.8                                                  d.sub.5                                                                              1.0000       14.6940                                                                           31.3743                                               d.sub.11                                                                             18.9434      8.4253                                                                            1.3056                                                d.sub.18                                                                             4.3238       2.0731                                                                            1.000                                                 __________________________________________________________________________    Aspherical Coefficients                                                                     13th surface                                                                  A.sub.4 = 0.14917 × 10.sup.-4                                           A.sub.6 = 0.27749 × 10.sup.-7                                           A.sub.8 = 0.87978 × 10.sup.-9                                           A.sub.10 = -0.39144 × 10.sup.-11                                        A.sub.12 = 0                                                                  21th surface                                                                  A.sub.4 = -0.69985 × 10.sup.-4                                          A.sub.6 = -0.22036 × 10.sup.-6                                          A.sub.8 = 0.14339 × 10.sup.-8                                           A.sub.10 = -0.22565 × 10.sup.-10                                        A.sub.12 = 0                                                                  (1) d.sub.4 /f.sub.w = 0.14                                                   (2) f.sub.12W /f.sub.T = -0.26                                                (3) AΔd.sub.3 /f.sub.w = 0.13                             __________________________________________________________________________    Example 3                                                                                f = 25.2 ˜ 50.0 ˜ 100.0                                           F.sub.NO = 4.54 ˜ 5.10 ˜ 5.60                                     f.sub.B = 33.312 ˜ 45.988 ˜ 61.636                                2 ω = 72.10° ˜ 37.26° ˜19.02.de               gree.                                                              r.sub.1 =                                                                        60.6491   d.sub.1 =                                                                        2.000                                                                              n.sub.d1 =                                                                       1.84666                                                                            υ.sub.d1 =                                                                23.78                                        r.sub.2 =                                                                        40.3467   d.sub.2 =                                                                        4.492                                                                              n.sub.d2 =                                                                       1.72916                                                                            υ.sub.d2 =                                                                54.68                                        r.sub.3 =                                                                        497.0114  d.sub.3 =                                                                        (Variable)                                                    r.sub.4 =                                                                        254.4307  d.sub.4 =                                                                        1.200                                                                              n.sub.d3 =                                                                       1.77250                                                                            υ.sub.d3 =                                                                49.60                                        r.sub.5 =                                                                        18.1680   d.sub.5 =                                                                        7.248                                                         r.sub.6 =                                                                        -36.0691  d.sub.6 =                                                                        1.200                                                                              n.sub.d4 =                                                                       1.77250                                                                            υ.sub.d4 =                                                                49.60                                        r.sub.7 =                                                                        63.3487   d.sub.7 =                                                                        0.200                                                         r.sub.8 =                                                                        41.5840   d.sub.8 =                                                                        2.734                                                                              n.sub.d5 =                                                                       1.84666                                                                            υ.sub.d5 =                                                                23.78                                        r.sub.9 =                                                                        -120.0723 d.sub.9 =                                                                        (Variable)                                                    r.sub.10 =                                                                       ∞ (Stop)                                                                          d.sub.10 =                                                                       1.000                                                         r.sub.11 =                                                                       24.9215 (Aspheric)                                                                      d.sub.11 =                                                                       2.860                                                                              n.sub.d6 =                                                                       1.51633                                                                            υ.sub.d6 =                                                                64.15                                        r.sub.12 =                                                                       -617.6101 d.sub.12 =                                                                       0.200                                                         r.sub.13 =                                                                       18.9826   d.sub.13 =                                                                       3.052                                                                              n.sub.d7 =                                                                       1.48749                                                                            υ.sub.d7 =                                                                70.21                                        r.sub.14 =                                                                       93.0258   d.sub.14 =                                                                       3.146                                                         r.sub.15 =                                                                       -18.9497  d.sub.15 =                                                                       1.200                                                                              n.sub.d8 =                                                                       1.84666                                                                            υ.sub.d8 =                                                                23.78                                        r.sub.16 =                                                                       -56.1226  d.sub.16 =                                                                       (Variable)                                                    r.sub.17 =                                                                       31.0270   d.sub.17 =                                                                       3.756                                                                              n.sub.d9 =                                                                       1.65844                                                                            υ.sub.d9 =                                                                50.86                                        r.sub.18 =                                                                       -29.3251  d.sub.18 =                                                                       2.735                                                         r.sub.19 =                                                                       -556.0150 (Aspheric)                                                                    d.sub.19 =                                                                       1.200                                                                              n.sub.d10 =                                                                      1.77250                                                                            υ.sub.d10 =                                                               49.60                                        r.sub.20 =                                                                       32.9380   d.sub.20 =                                                                       (Variable)                                                    r.sub.21 =                                                                       49.0324   d.sub.21 =                                                                       1.200                                                                              n.sub.d11 =                                                                      1.51633                                                                            υ.sub.d11 =                                                               64.15                                        r.sub.22 =                                                                       37.1931                                                                    __________________________________________________________________________    Zooming Spaces                                                                f      25.2         50.0                                                                              100.8                                                 d.sub.3                                                                              1.2000       13.4747                                                                           30.3865                                               d.sub.9                                                                              25.1511      10.3081                                                                           1.2783                                                d.sub.16                                                                             4.2768       2.0581                                                                            1.0000                                                d.sub.20                                                                             1.0000       3.2183                                                                            4.2776                                                __________________________________________________________________________    Aspherical Coefficients                                                                     11th surface                                                                  A.sub.4 = 0.21918 × 10.sup.-4                                           A.sub.6 = -0.29023 × 10.sup.-7                                          A.sub.8 = 0.44504 × 10.sup.-5                                           A.sub.10 = -0.59894 × 10.sup.-10                                        A.sub.12 = 0.31765 × 10.sup.-12                                         19th surface                                                                  A.sub.4 = -0.77374 × 10.sup.-4                                          A.sub.6 = -0.12648 × 10.sup.-6                                          A.sub.8 = -0.20680 × 10.sup.-8                                          A.sub.10 = 0.23110 × 10.sup.-0                                          A.sub.12 = -0.11142 × 10.sup.-12                                        (1) d.sub.4 /f.sub.w = 0.11                                                   (2) f.sub.12W /f.sub.T = -0.30                                                (3) AΔd.sub.3 /f.sub.w = 0.13                             __________________________________________________________________________    Example 4                                                                                f = 25.4 ˜ 51.7 ˜ 106.9                                           F.sub.NO = 4.50 ˜ 5.06 ˜ 5.70                                     f.sub.B = 34.854 ˜ 49.843 ˜ 61.773                                2 ω = 72.01° ˜ 35.93° ˜                       17.69°                                                      r.sub.1 =                                                                        247.0706  d.sub.1 =                                                                        2.400                                                                              n.sub.d1 =                                                                       1.80518                                                                            υ.sub.d1 =                                                                25.43                                        r.sub.2 =                                                                        74.0393   d.sub.2 =                                                                        4.216                                                                              n.sub.d2 =                                                                       1.48749                                                                            υ.sub.d2 =                                                                70.21                                        r.sub.3 =                                                                        -184.0125 d.sub.3 =                                                                        0.200                                                         r.sub.4 =                                                                        49.0559   d.sub.4 =                                                                        3.154                                                                              n.sub.d3 =                                                                       1.72916                                                                            υ.sub.d3 =                                                                54.68                                        r.sub.5 =                                                                        166.3259  d.sub.5 =                                                                        (Variable)                                                    r.sub.6 =                                                                        407.8742  d.sub.6 =                                                                        1.500                                                                              n.sub.d4 =                                                                       1.77250                                                                            υ.sub.d4 =                                                                49.60                                        r.sub.7 =                                                                        16.0577   d.sub.7 =                                                                        5.394                                                         r.sub.8 =                                                                        -28.1958  d.sub.8 =                                                                        1.200                                                                              n.sub.d5 =                                                                       1.77250                                                                            υ.sub.d5 =                                                                49.60                                        r.sub.9 =                                                                        54.9009   d.sub.9 =                                                                        0.200                                                         r.sub.10 =                                                                       37.5539   d.sub.10 =                                                                       2.873                                                                              n.sub.d6 =                                                                       1.84666                                                                            υ.sub.d6 =                                                                23.78                                        r.sub.11 =                                                                       -78.6995  d.sub.11 =                                                                       (Variable)                                                    r.sub.12 =                                                                       ∞ (Stop)                                                                          d.sub.12 =                                                                       0.800                                                         r.sub.13 =                                                                       38.1316 (Aspheric)                                                                      d.sub.13 =                                                                       2.511                                                                              n.sub.d7 =                                                                       1.56016                                                                            υ.sub.d7 =                                                                60.30                                        r.sub.14 =                                                                       -103.8484 d.sub.14 =                                                                       0.200                                                         r.sub.15 =                                                                       22.4272   d.sub.15 =                                                                       2.680                                                                              n.sub.d8 =                                                                       1.48749                                                                            υ.sub.d8 =                                                                70.21                                        r.sub.16 =                                                                       431.8811  d.sub.16 =                                                                       2.118                                                         r.sub.17 =                                                                       -26.4421  d.sub.17 =                                                                       1.200                                                                              n.sub.d9 =                                                                       1.80518                                                                            υ.sub.d9 =                                                                25.43                                        r.sub.18 =                                                                       -192.5764 d.sub.18 =                                                                       (Variable)                                                    r.sub.19 =                                                                       31.9279   d.sub.19 =                                                                       3.431                                                                              n.sub.d10 =                                                                      1.60311                                                                            υ.sub.d10 =                                                               60.68                                        r.sub.20 =                                                                       -36.8010  d.sub.20 =                                                                       7.027                                                         r.sub.21 =                                                                       ∞ (Aspheric)                                                                      d.sub.21 =                                                                       1.212                                                                              n.sub.d11 =                                                                      1.79952                                                                            υ.sub.d11 =                                                               42.24                                        r.sub.22 =                                                                       43.9262                                                                    __________________________________________________________________________    Zooming Spaces                                                                f      25.4         51.7                                                                              106.9                                                 d.sub.5                                                                              1.0000       13.9486                                                                           30.7176                                               d.sub.11                                                                             21.2889      9.6535                                                                            1.5000                                                d.sub.18                                                                             5.9891       2.6387                                                                            1.0000                                                __________________________________________________________________________    Aspherical Coefficients                                                                     13th surface                                                                  A.sub.1 = 0.83634 × 10.sup.-5                                           A.sub.6 = 0.11592 × 10.sup.-7                                           A.sub.8 = 0.68945 × 10.sup.-10                                          A.sub.10 = 0.47517 × 10.sup.-12                                         A.sub.12 = 0                                                                  21st surface                                                                  A.sub.4 = -0.52001 × 10.sup.-4                                          A.sub.6 = -0.15982 × 10.sup.-6                                          A.sub.8 = 0.43993 × 10.sup.-10                                          A.sub.10 = -0.15466 × 10.sup.-13                                        A.sub.12 = 0                                                                  (1) d.sub.4 /f.sub.w = 0.28                                                   (2) f.sub.12W /f.sub.T = -0.24                                                (3) AΔd.sub.3 /f.sub.w = 0.20                             __________________________________________________________________________

According to the present invention as hereinbefore explained at greatlength, there is provided a positive lens group preceding type of zoomlens system including a second lens group having a negative refractingpower and consisting of three lenses, wherein an aspheric surface orsurfaces are effectively used whereby various aberrations introduced byincreasing the refracting power of each lens group, so that while highoptical properties are maintained, the number of lenses involved can bereduced to achieve cost reductions and compactness. In this zoom lenssystem, the focal distance at the wide-angle end is shorter than thediagonal length of the image pickup surface and the zoom ratio is atleast 3, with well-corrected aberrations. Thus, the compact zoom lenssystem according to the present invention is suitable for use on stillcameras.

What is claimed is:
 1. A zoom lens system, in order from an object to animage side, comprising:a first lens group having a positive power; asecond lens group having a negative power; a third lens group having apositive power; and a fourth lens group having a positive power, whereinsaid first, second, third, and fourth lens groups are moved along anoptical axis to effect zooming from a wide-angle end to a telephoto endsuch that all separations reserved between adjacent said lens groupsvary during zooming, said second lens group is comprised of less thanfour lenses, said fourth lens group has at least a positive lenscomponent and a negative lens component, said third and fourth lensgroups combined have at least two aspheric surfaces and are furthercharacterized by conforming to the following conditional inequality:

    0.1<d.sub.4 /f.sub.W <0.5

where d₄ is an air separation length between the positive and negativelens components of said fourth lens group and f_(W) is a composite focaldistance of the zoom lens system at the wide-angle end, and wherein saidfirst lens group, said third lens group and said fourth lens group aremoved toward the object side during zooming from the wide-angle end tothe telephoto end, while the position of said second lens group at thetelephoto end is located on the object side relative to the position ofsaid second lens group at the wide-angle end.
 2. A zoom lens systemaccording to claim 1, wherein the composite focal distance of the zoomlens system at the wide-angle end is shorter than a diagonal length ofan image pickup surface.
 3. A zoom lens system, in order from an objectto an image side, comprising:a first lens group having a positive power;a second lens group having a negative power; a third lens group having apositive power; and a fourth lens group having a positive power, whereinsaid first, second, third, and fourth lens groups are movably disposedalong an optical axis to effect zooming from a wide-angle end to atelephoto end, said second lens group is comprised of less than fourlenses, said fourth lens group has at least a positive lens componentand a negative lens component, said third and fourth lens groupscombined have at least two aspheric surfaces and are furthercharacterized by conforming to the following conditional inequality:

    0.1<d.sub.4 /f.sub.W <0.5

where d₄ is an air separation length between the positive and negativelens components of said fourth lens group and f_(W) is a composite focaldistance of the zoom lens system at the wide-angle end, and said thirdlens group comprises, in order from the object side, a positive lens, apositive lens and a negative lens.
 4. A zoom lens system according toclaim 3, wherein the composite focal distance of the zoom lens system atthe wide-angle end is shorter than the diagonal length of an imagepickup surface.
 5. A zoom lens system according to claim 3, wherein saidthird lens group comprises, in order from the object side, adouble-convex lens with a radius of curvature of an object-side surfacethereof being smaller than that of an image-side surface thereof, apositive lens and a negative meniscus lens convex on the image side. 6.A zoom lens system, in order from an object to an image side,comprising:a first lens group having a positive power; a second lensgroup having a negative power; a third lens group having a positivepower; and a fourth lens group having a positive power, wherein saidfirst, second, third, and fourth lens groups are movably disposed alongan optical axis to effect zooming from a wide-angle end to a telephotoend, said second lens group is comprised of less than four lenses, saidfourth lens group has at least a positive lens component and a negativelens component, said third and fourth lens groups combined have at leasttwo aspheric surfaces and are further characterized by conforming to thefollowing conditional inequality:

    0.1<d.sub.4 /f.sub.W <0.5

where d₄ is an air separation length between the positive and negativelens components of said fourth lens group and f_(W) is a composite focaldistance of the zoom lens system at the wide-angle end, and a lens ofsaid third lens group proximate to the object side and a second lens ofsaid fourth lens group as viewed from the object side have asphericsurfaces.
 7. A zoom lens system according to claim 6, wherein thecomposite focal distance of the zoom lens system at the wide-angle endis shorter than a diagonal length of an image pickup surface.
 8. A zoomlens system, in order from an object to an image side, comprising:afirst lens group having a positive power; a second lens group having anegative power; a third lens group having a positive power; and a fourthlens group having a positive power, wherein said first, second, third,and fourth lens groups are moved along an optical axis to effect zoomingfrom a wide-angle end to a telephoto end such that all separationsreserved between adjacent said lens groups vary during zooming, saidsecond lens group is comprised of less than four lenses, said fourthlens group has at least a positive lens component and a negative lenscomponent, said third and fourth lens groups combined have at least twoaspheric surfaces and are further characterized by conforming to thefollowing conditional inequality:

    0.1<d.sub.4 /f.sub.W <0.5

where d₄ is an air separation length between the positive and negativelens components of said fourth lens group and f_(W) is a composite focaldistance of the zoom lens system at the wide-angle end, and said firstlens group and said second lens group are moved toward the object sideas an integral piece for the purpose of focusing.
 9. A zoom lens systemaccording to claim 8, wherein the composite focal distance of the zoomlens system at the wide-angle end is shorter than a diagonal length ofan image pickup surface.
 10. A zoom lens system, in order from an objectto an image side, comprising:a first lens group having a positive power;a second lens group having a negative power; a third lens group having apositive power; and a fourth lens group having a positive power, whereinsaid first, second, third, and fourth lens groups are moved along anoptical axis to effect zooming from a wide-angle end to a telephoto endsuch that all separations reserved between adjacent said lens groupsvary during zooming, said second lens group is comprised of less thanfour lenses, said fourth lens group has at least a positive lenscomponent and a negative lens component, said third and fourth lensgroups combined have at least two aspheric surfaces and are furthercharacterized by conforming to the following conditional inequality: 0.1<d₄ /f_(W) <0.5where d₄ is an air separation length between thepositive and negative lens components of said fourth lens group andf_(W) is a composite focal distance of the zoom lens system at thewide-angle end, and said third lens group comprises, in order from theobject side, a positive lens, a positive lens and a negative lens.
 11. Azoom lens system according to claim 10, wherein the composite focaldistance of the zoom lens system at the wide-angle end is shorter than adiagonal length of an image pickup surface.
 12. A zoom lens systemaccording to claim 10 or 11, wherein said third lens group comprises, inorder from the object side, a double-convex lens with a radius ofcurvature of an object-side surface thereof being smaller than that ofan image-side surface thereof, a positive lens and a negative meniscuslens convex on the image side.
 13. A zoom lens system, in order from anobject to an image side, comprising:a first lens group having a positivepower; a second lens group having a negative power; a third lens grouphaving a positive power; and a fourth lens group having a positivepower, wherein said first, second, third, and fourth lens groups aremoved along an optical axis to effect zooming from a wide-angle end to atelephoto end such that all separations reserved between adjacent saidlens groups vary during zooming, said second lens group is comprised ofless than four lenses, said fourth lens group has at least a positivelens component and a negative lens component, said third and fourth lensgroups combined have at least two aspheric surfaces and are furthercharacterized by conforming to the following conditional inequality: 0.1<d₄ /f_(W) <0.5where d₄ is an air separation length between thepositive and negative lens components of said fourth lens group andf_(W) is a composite focal distance of the zoom lens system at thewide-angle end, said fourth lens group has a positive lens component anda negative lens component in order from the object side, and said fourthlens group has, in order from the object side, a double-convex lens anda double-concave lens with a radius of curvature of an image-sidesurface thereof being smaller than that of an object-side surfacethereof.
 14. A zoom lens system according to claim 13, wherein thecomposite focal distance of the zoom lens system at the wide-angle endis shorter than a diagonal length of an image pickup surface.
 15. A zoomlens system, in order from an object to an image side, comprising:afirst lens group having a positive power; a second lens group having anegative power; a third lens group having a positive power; and a fourthlens group having a positive power, wherein said first, second, third,and fourth lens groups are moved along an optical axis to effect zoomingfrom a wide-angle end to a telephoto end such that all separationsreserved between adjacent said lens groups vary during zooming, saidsecond lens group is comprised of less than four lenses, said fourthlens group has at least a positive lens component and a negative lenscomponent, said third and fourth lens groups combined have at least twoaspheric surfaces and are further characterized by conforming to thefollowing conditional inequality:
 0. 1<d₄ /f_(W) <0.5where d₄ is an airseparation length between the positive and negative lens components ofsaid fourth lens group and f_(W) is a composite focal distance of thezoom lens system at the wide-angle end, said first lens group iscomprised of less than four lenses, said third lens group is comprisedof less than four lenses, and said fourth lens group is comprised ofless than four lenses, and a lens of said third lens group proximate tothe object side and a second lens of said fourth lens group as viewedfrom the object side have aspheric surfaces.
 16. A zoom lens systemaccording to claim 15, wherein the composite focal distance of the zoomlens system at the wide-angle end is shorter than a diagonal length ofan image pickup surface.